The term “spectrum analyzer” refers to a device used to examine the spectral composition of a radio frequency (RF) input signal. A spectrum analyzer typically comprises components including an attenuator to reduce the RF input signal amplitude; one or more mixers and one or more local oscillators (LO) to convert the RF input signal to an intermediate frequency (IF) signal; and a system of filtering and measuring the resultant IF signal. A spectrum analyzer may use several frequency bands to cover a broad frequency spectrum, analyzing one band at a time.
When spectrum analyzers convert the RF signal to an IF signal there is attenuation, an amplitude loss (or gain), which must be corrected in hardware or software before the signal amplitude is displayed. This amplitude loss can be thought of as the sum of the losses through the various elements in the signal path, such as mixers and filters, where the amplitude loss from each element is dependent on the frequency of the signal passing through it. The attenuation vs. frequency for these elements often changes significantly with temperature.
Many modern spectrum analyzers use the Fast Fourier Transform (FFT) technique to convert time-domain signal data into frequency-domain signal data. Processing high-resolution FFTs quickly requires a powerful processor, and depends on the correct utilization of processor resources, such as parallel processing across multiple processor cores, and utilization of Single Instruction, Multiple Data (SIMD) instructions, such as Advanced Vector Extensions (AVX). The SIMD instructions for modern processors have recently made tremendous advances, including the introduction of Fused Multiply Accumulate, promising to advance the signal processing capabilities of a desktop or laptop Personal Computer (PC) even further.
Handheld spectrum analyzers contain fewer, simpler, and less accurate components and consume less power than traditional portable or bench top spectrum analyzers. They typically have a Liquid Crystal Display (LCD) and buttons for a user interface. They are generally not capable of processing automation commands and are of minimal usefulness in a lab setting. They usually have slower processors which are not capable of quickly processing very large FFTs, and are therefore of limited usefulness regarding real-time spectrum analysis. Their processors are usually fixed and cannot be readily upgraded.
A key specification for modern spectrum analysis is the real-time streaming bandwidth (RTS bandwidth), the amount of RF spectrum which can be simultaneously acquired and analyzed. This RTS bandwidth is limited by the Nyquist frequency and several other factors, limiting it to less than ½ of the sample rate, with a more practical limit around ⅓ of the sample rate or less. Therefore, the RTS bandwidth is limited by the number of samples per second which can be transferred to the processor.
The amplitude accuracy of the data returned is dependent on knowing the path loss for any given temperature. If an IF filter has passband ripple of 1.5 dB, characterizing the shape of this ripple is necessary to properly correct for it.
Owning a modern spectrum analyzer capable of streaming 20 MHz or more of real-time spectrum is currently cost prohibitive for many small businesses, students, and inventors. What is needed is a low-cost, low-power, lightweight, portable spectrum analyzer similar in size and weight to a traditional RF power sensor, that is capable of correcting for filter and mixer loss across its operating temperature with minimal additional circuitry, combined with the signal processing power of the modern personal computer, and that will benefit from newer, more powerful PC/laptop processors as they become available.